Apple ProDOS

ProDOS 8

Developer	Apple Computer
OS family	SOS
Working state	Historic
Source model	Closed source
Initial release	October 1983
Latest release	2.0.3 / May 6, 1993
Available in	English
Platforms	Apple II series of personal computers
Kernel type	Monolithic kernel
Default user interface	primarily text user interface
License	Apple Software License Agreement

ProDOS 16

Developer	Apple Computer
OS family	SOS
Working state	Historic
Source model	Closed source
Initial release	September 1986
Latest release	1.6 / June 14, 1988
Available in	English
Platforms	Apple IIGS
Kernel type	Monolithic kernel
License	Apple Software License Agreement

ProDOS was the name of two similar operating systems for the Apple II series of personal computers. The original ProDOS, renamed ProDOS 8 in version 1.2, was the last official operating system usable by all Apple II series computers, and was distributed from 1983 to 1993. The other, ProDOS 16, took advantage of the extra capabilities of the 16-bit Apple IIGS, but was short-lived and replaced by GS/OS within a year.

ProDOS was marketed by Apple as meaning Professional Disk Operating System, and became the most popular operating system for the Apple II series of computers 10 months after its release in January 1983.

Background

ProDOS was released to address shortcomings in the earlier Apple operating system (called simply DOS), which was beginning to show its age. DOS only had built-in support for 5.25" floppy disks and required patches to use peripheral devices such as hard disk drives and non-Disk II floppy disk drives, including 3.5" floppy drives. ProDOS added a standard method of accessing ROM-based drivers on expansion cards for disk devices, expanded the maximum volume size from about 400 kilobytes to 32 megabytes, introduced support for hierarchical subdirectories (a vital feature for organizing a hard disk's storage space), and supported RAM disks on machines with 128kB or more of memory. ProDOS addressed problems with handling hardware interrupts, and included a well-defined and documented programming and expansion interface, which DOS had always lacked. Although ProDOS also included support for a real-time clock (RTC), this support went largely unused until the release of the Apple IIGS, the first in the Apple II series to include an RTC on-board. Third-party clocks were available for the II Plus, IIe, and IIc, however.

ProDOS, unlike earlier Apple DOS versions, had its developmental roots in SOS, the operating system for the ill-fated Apple III computer released in 1980. Pre-release documentation for ProDOS (including early editions of Beneath Apple ProDOS) documented SOS error codes, notably one for switched disks, that ProDOS itself could never generate. Its disk format and programming interface were completely different from those of DOS, and ProDOS could not read or write DOS 3.3 disks except by means of a conversion utility; while the low-level track-and-sector format of DOS 3.3 disks was retained for 5.25 inch disks, the high-level arrangement of files and directories was completely different. For this reason, most machine-language programs that ran under DOS would not work under ProDOS. However, most BASIC programs would work, though they sometimes required minor changes. A third-party program called DOS.MASTER enabled users to have multiple virtual DOS 3.3 partitions on a larger ProDOS volume.

With the release of ProDOS came the end of support for Integer BASIC and the original Apple II model, which had long since been effectively supplanted by Applesoft BASIC and the Apple II Plus. Whereas DOS 3.3 always loaded built-in support for BASIC programming, under ProDOS this job was given to a separate system program called BASIC.SYSTEM, which one launched to run and write Applesoft BASIC programs. BASIC itself continued to be built into the Apple ROMs; BASIC.SYSTEM was merely a command interpreter enhancement that allowed BASIC programs to access ProDOS by means of the same "Control-D" text output they had used under DOS 3.3. BASIC.SYSTEM alone required about as much memory as the whole of DOS 3.3. Since the ProDOS kernel itself was stowed away in the "Language Card" RAM, the usable amount of RAM for BASIC programmers remained the same under ProDOS as it had been under DOS 3.3.

Despite ProDOS's many advantages, many users and programmers resisted it for a time because of their investment in learning the ins and outs of DOS and in DOS-based software and data formats. A contributing reason was that ProDOS allowed only fifteen characters in a filename compared to DOS's thirty. But Apple's integrated software package AppleWorks, released in 1984, proved a compelling reason to switch, and by the end of 1985 few new software products were being released for the older operating system. Apple IIs continued to be able to boot DOS (even the Apple IIGS could boot DOS floppies) but as 3.5" floppies and hard disks became more prevalent, most users spent the bulk of their time in ProDOS.

The Apple IIe, also released in 1983, was the first Apple II computer to have 64kB of memory built in. For a while, Apple shipped both DOS 3.3 and ProDOS with new computers.

ProDOS was renamed ProDOS 8 when ProDOS 16 was released to support the 16-bit Apple IIGS computer, although ProDOS 16 was soon replaced by GS/OS.

Requirements

All editions of ProDOS require an Apple II series computer.

ProDOS 8 requires 64kB of memory to run. The original ProDOS (8) 1.0 through 1.0.2 required only 48kB for the kernel, but nearly all programs, including the BASIC.SYSTEM needed to use Applesoft BASIC, required 64kB, making a 48kB system useless for ProDOS as a practical matter, and support for 48kB machines was removed in version 1.1.

ProDOS 8 version 2.x requires a 65C02 or later (65802, 65816) CPU. ProDOS 8 2.x runs in 64kB, but the utility programs on the system disk require 128kB. Systems with a 6502 CPU instead of a 65C02 must use ProDOS 8 versions prior to version 2.0.

ProDOS 16 requires an Apple IIGS.

Availability

ProDOS system disk images can be downloaded legally from a number of user group web sites. It can also be purchased on disk from Syndicomm, which distributes it under license from Apple Computer.

Disk support

ProDOS 8 natively supports Disk II-compatible floppy drives, a RAM drive of approximately 59kB, and "intelligent" devices with a standardized method of accepting block reads and writes. Specialized block device drivers can be hooked into the OS as well.

Filesystem

ProDOS uses the same filesystem as the earlier Apple SOS for the Apple III. The SOS/ProDOS filesystem is native to Apple SOS, ProDOS 8, ProDOS 16, and GS/OS. Some Mac OS versions also come with a file system translator to handle this filesystem.

A volume is allocated in 512 byte blocks. It can can be up to 32 megabytes. Each file can be up to 16 megabytes. Each volume (disk or hard drive partition) has a "volume name", a filename which is used as the base directory name; having two volumes with the same volume name could result in conflicts. If necessary, ProDOS would search all available drives to find a named volume. Subdirectories are supported, and the concept of a "prefix" (working directory or current path) was provided to make working with subdirectories easier.

File, directory, and volume names can be 1 to 15 characters, starting with a letter then containing letters, digits, or periods. Each file entry also contains the 16 bit (2 byte) pointer to the block containing the beginning of the file (or its block index); a 16 bit block count; a 24 bit (3 byte) file size; an 8 bit (1 byte) filetype; a 16 bit auxiliary type (the meaning of which depends upon the filetype); creation and modification timestamps; and data related to how the file is stored on the volume. Sparse files are supported. The volume header contains similar information as relevant to volumes.

Directories (including the root directory) are sequentially indexed, with each block starting with the addresse of the previous block (or zero if none) and the subsequent block (or zero if none). The root directory on most disks is initialised to 4 blocks, allowing 51 entries (excluding the volume header). It never changes in size, except by manual intervention with special tools. Subdirectories begin at one block, and grow automatically as needed.

Normal files are progressively indexed. Single block files (under 513 bytes) have no index block; the directory entry points directly to the block of file data. Files with between 2 and 256 blocks (513 bytes to 128 kB) of data have a single index block, to which the directory entry points, which contains a list of up to 256 data block addresses. Larger files have a master index block containing a list of up to 256 index block addresses.

The volume has a bitmap of used blocks. Other than this there is no central file allocation table.

Almost every ProDOS 8 volume has a boot sector that supports both ProDOS and SOS. This allows a disk to be used to boot on either Apple II or Apple III computers by putting both operating system kernels in the top directory: the Apple II boot sector looks for the file PRODOS and the Apple III boot sector looks for the file SOS.KERNEL.

ProDOS has no kernel support for other filesystems. If necessary, a conversion utility on the main system disk is used to transfer files individually between ProDOS and older Apple DOS 3.3 disks.

References

• Don Worth and Pieter Lechner. Beneath Apple ProDOS. Quality Software, 1984. ISBN 0-912985-05-4

External links

• Apple II History: DOS 3.3, ProDOS & Beyond
• Apple II Information Reference - Apple2.info - ProDOS 8 Technical Reference Manual, wiki formatted, with FAQs, compatibility guides, etc.
• ProDOS 8 Technical Reference Manual - HTML formatted
• A2Central.com – Apple II news and downloads